Nanotube System May Improve Tumor Detection

A newly developed method called spectral triangulation system can pinpoint tumors with carbon nanotubes.

A study published in Nanoscale discussed the highly sensitive detector system, which relies on carbon nanotubes that naturally fluoresce at short-wave infrared wavelengths when hit with visible light. By using light from an LED array and an InGaAs (indium gallium arsenide) avalanche photodiode, the system can read faint signals coming from the nanotubes, which are up to 20 millimeters deep within the lab’s simulated tissue.

“This avalanche photodiode can count photons in the short-wave infrared, which is a challenging spectral range for light sensors,” said Bruce Weisman, developer of the system. “The main goal is to see how well we can detect and localize emission from very small concentrations of nanotubes inside biological tissues. This has potential applications in medical diagnosis.”

Although using LEDs is unconventional, it is both inexpensive and effective.

“It's relatively unconventional to use LEDs,” Weisman said. “Instead, lasers are commonly used for excitation, but laser beams can't be focused inside tissues because of scattering. We bathe the surface of the specimen in unfocused LED light, which diffuses through the tissues and excites nanotubes inside.”

The system works by using an optical probe that is small in size and mounted to the frame of a 3D printer. This probe follows a computer programmed pattern as it gently touches the skin, making readings at grid points that are spaced a few millimeters apart.

Light from the nanotubes are partially absorbed by water as it travels through tissues before it reaches the detector.

“A 2-dimensional search tells us the emitter's X and Y coordinates but not Z -- the depth,” Weisman said. “That's a very difficult thing to deduce from a surface scan. We make use of the fact that different wavelengths of nanotube emission are absorbed differently going through tissues.”

The water that surrounds the tissue is able to absorb the longer wavelengths from the nanotubes more strongly than the shorter wavelengths.

“If we're detecting nanotubes close to the surface, the long and the short wavelength emissions are relatively similar in intensity,” Weisman said. “We say the spectrum is unperturbed. But if the emission source is deeper, water in that tissue absorbs the longer wavelengths preferentially to the shorter wavelengths. So the balance between the intensities of the short and long wavelengths is a yardstick to measure how deep the source is. That's how we get the Z coordinate.”

At this time, the detector is being tested in Robert Bast’s lab, who is an ovarian cancer expert and vice president for translational research at the University of Texas MD Anderson Cancer Center.

“It gives us a fighting chance to see nanotubes deeper inside tissues because so little of the light that nanotubes emit finds its way to the surface,” Weisman said. “We've been able to detect deeper into the tissues than I think anyone else has reported.”